1. Field of the Invention
This invention relates to rubber compositions for tire treads, which are well balanced in the performances of rolling resistance, sliding coefficient of friction on wet road surfaces and wear resistance.
2. Prior Art
Conventionally, for the sake of the reduction of fuel consumption, various investigations have been made in order to obtain rubber compositions for tire treads which are balanced in both performances of the rolling resistance and the sliding coefficient of friction on wet road surfaces.
For example, Japanese patent application laid open under No. 60-255838 discloses rubber compositions containing 20 wt % of conjugated diolefin-vinyl aromatic hydrocarbon random copolymer which is copolymerized using an organic lithium polyfunctional initiator and coupled using a compound containing a halogenated tin compound, wherein the content of said vinyl aromatic hydrocarbon is 3-60 wt %, the content of tin bonded to the copolymer is 400 ppm or more and the Mooney viscosity (ML1+4, 100.degree. C.) is 20-200.
Japanese patent application laid open under No. 61-14214 discloses improved rubber polymers containing a rubbery conjugated dien compound polymer or a rubbery vinyl aromatic compound-conjugated dien compound copolymer, wherein 10 wt % or more of said polymer or copolymer is coupled using a tri- or more functional coupling agent and 20 wt % or more is added a mono- or difunctional active tin compound and 30 wt % or more of molecules have an active tin compound bonded.
Although both prior arts intend to enhance the balance in rolling resistance and wet skid performance using a copolymer coupled by a functional tin compound and defining the content of tin etc., a high level of balance has not been attained yet.
The inventors of the present invention disclose rubber compositions in Japanese patent applications laid open under No. 60-26044 and No. 60-26045 which improved the balance in the performances of rolling resistance, sliding coefficient of friction on wet road surfaces and wear resistance by compounding a specific carbon black defined by the correlation between N2 SA and IA as a reinforcement in a rubber composition which was coupled using a tin tetrachloride.
It is true that our said prior invention attained well balanced performances of rolling resistance, sliding coefficient of friction on wet road surfaces and wear resistance, but the processability and workability which are very important in tire manufacture were not fully satisfactory.